In the production of minerals, e.g., oil and gas, certain lithological properties of a subterranean reservoir must be determined. Two of the most important of these properties are the porosity and pore compressibility of the reservoir. Porosity of a material is the ratio of the aggregate volume of its void or pore spaces (i.e., pore volume) to its gross bulk volume and, in the case of an oil or gas reservoir, is a measure of the capacity within the reservoir rock which is available for storing oil or gas. Normally, porosity is determined by taking core samples from the reservoir and carrying out well-defined measurement techniques on the samples. There are several techniques available for making such measurements, many of which are described in PETROLEUM PRODUCTION ENGINEERING--DEVELOPMENT by L. D. Uren, Fourth Edition, McGraw-Hill Book Company, Inc., 1956, pps. 660-669. Another standard reference is American Petroleum Institute, API Recommended Practice for Core-Analysis Procedure, API RP 40, 1960, 55 pp. Such porosity measurements are performed on the core samples at ambient pressure conditions. However, the effect of overburden pressure or confining stress on the porosity of a sample of a reservoir rock is an important factor in determining oil and gas reserves, as well as in estimating reservoir performance. Accurate determinations of pore volume compressibility are needed for correcting core sample porosity to reservoir stress conditions and for estimating the changes in reservoir pore volume due to pressure depletion. Such a pore volume compressibility determination has been routinely carried out by a fluid expulsion procedure known as "brine squeeze-out". In this procedure a cylindrically-shaped sample of known pore volume is saturated with brine and placed in a rubber-sleeved core holder. Confining stress is applied at selected values of sleeve pressure, and the brine is allowed to flow from one face of the sample into a pipette open to atmospheric pressure. The change in pore volume is considered to be the amount of brine expelled at any particular confining pressure. For consolidated rock samples the reference pressure, representing zero stress, is typically 200 to 450 psi. This is presumably the pressure at which the rubber sleeve fits snugly around the sample. Therefore, any brine expelled at higher pressures indicates actual pore volume change and not the volume of brine displaced from between the rock and the rubber sleeve. The pore volume at the "zero stress" condition is presumed to be the same as the pore volume measured at ambient conditions outside of the core holder.
Two types of reservoir rocks, accounting for a large fraction of reserves, create difficulties when interpreting data from the brine squeeze-out. Unconsolidated sands tend to compress significantly at very low stresses. In fact, the confining pressure required to seat the rubber sleeve can result in the loss of several percent of the ambient pore volume. The reference pressure chosen for tests with unconsolidated sand may not be the zero stress condition that is required. This condition would lead to the underestimation of porosity correction. The other type of sample which causes difficulty is characterized by vuggy porosity or very coarse grain size, i.e., rocks with large pores. At low pressures, the stiffness of the rubber sleeve presents the sleeve from invading the large surface pores. At increasing pressure, however, the sleeve invades these pores and brine is displaced which is not attributable to an actual reduction of pore volume. This would result in an apparent porosity loss which is much higher than the actual value.
Consequently, there is a crucial need for porosity and pore compressibility measurements under reservoir confining stress conditions so that accurate calculation of reserves and changes in reserves due to production depletion can be made. It is this need for core porosity and pore volume compressibility measurements at reservoir stress conditions that the present invention is directed.